The present invention relates generally to an exposure apparatus, and more particularly to an immersion exposure apparatus.
A projection exposure apparatus has been conventionally employed which exposes a circuit pattern of a reticle (mask) onto a wafer via a projection optical system. Recently, an exposure apparatus having a higher resolution, throughput, and transfer precision has been increasingly demanded. The immersion exposure is one attractive means to meet the demand for the high resolution.
The immersion exposure uses a liquid (immersion material) for a medium at the wafer side of the projection optical system, promoting a high numerical aperture (NA) of the projection optical system. The NA of the projection optical system is defined as NA=n×sin θ, where n is a refractive index of the medium. The NA increases up to “n” when a medium having a refractive index higher than the air's refractive index, i.e., n>1 is filled in the space between the projection optical system and the wafer. The immersion exposure intends to reduce the resolution R of the exposure apparatus which is defined as R=k1×(λ/NA), where k1 is a process constant, and λ is a wavelength of a light source.
In the immersion exposure, a local fill system is proposed which locally fills the liquid in a space between the final lens of the projection optical system and the wafer. See, for example, Japanese Patent Application, Publication No. 2005-150290.
While the conventional immersion exposure apparatus uses water for the immersion material, the immersion material having a higher refractive index is being developed to meet the demand for finer processing. National Institute of Standards and Technology (“NIST”) proposes some immersion materials each having a higher refractive index than that of water, such as inorganic acid or salt added water (e.g., H3PO4) and alcohol derivatives (e.g., 1,2,3-propanetriol). Mitsui Chemicals, Inc. and JSR Corporation propose to use flammable hydrocarbon organic liquid for the immersion material having a high refractive index. This immersion material when used for a conventional immersion exposure apparatus is likely to burn due to ignition sources provided in the immersion exposure apparatus, such as an electric circuit, a motor, and an exoergic component.